Mammalian skin and its appendages function as the outermost barrier of the body to protect inner organs from environmental hazards and keep essential fluids within the body. Homeostasis and integrity of mammalian skin are maintained by multiple progenitor and stem cell populations residing in distinct skin compartments. In particular, multipotent hair follicle stem cells (HFSCs) located in the bulge region maintain hair follicles (HFs) during normal hair cycle and epidermal wound healing. Understanding molecular mechanisms that regulate self-renewal and activation of HFSCs is a fundamental prerequisite to exploit these fascinating cells for regenerative medicine. Among the mechanisms, microRNAs (miRNAs) are a class of small, noncoding RNAs that take essential roles in mammalian gene regulation in diverse cell types and tissues. Unlike transcriptional mechanisms that potently activate or suppress gene expression, miRNAs regulate protein production of mRNAs at post-transcriptional levels and usually function to fine tune the output of the transcriptome. Despite modest regulation of individual targets, miRNAs broadly modulate a large number (60%) of genes and play important roles in a wide range of biological processes. In mammalian skin, the critical functions of the entire miRNA pathway in both embryonic skin development and maintenance of adult HF lineages have been well appreciated. In sharp contrast, there is no report identifying any individual miRNA that is required for the self-renewal of the HFSCs in adult skin. Consequently, the underlying mechanism of miRNA-mediated regulation in the HFSCs remains largely unknown. To address these important issues, we propose to focus on miR-205, the most highly expressed and functionally important miRNA in the HFSCs, and examine its roles in self- renewal and migration of adult HFSCs (Aim 1 and 2). To provide mechanistic insights for these functions, we will apply biochemical purification of miRISC and its associated mRNA targets to the WT and miR-205 knockout HFSCs and identify all miR-205-associated targets in an HFSC- specific manner. The long-term goal of this project is to elucidate individual miRNAs' functions and provide mechanistic insights to these molecules during homeostasis and stressed conditions including injury response and tumorigenesis in the HFSCs. Taken together, studies proposed here, if successful, will significantly enhance our knowledge about individual miRNAs' functions in adult HFSCs. The knowledge gained from these studies under normal and stressed conditions will pave the way to manipulate miRNAs and utilize these stem cells for regenerative medicine.